New Mechanism of Ionic Conductivity in Hydrogen-Bonded Crystals M3H(XO4)2[M=Rb, Cs, X=S, Se]

A model for the mechanism of ionic conductivity in the high temperature paraelastic phase of M 3 H(XO 4 ) 2 [M=Rb, Cs, X=S, Se] type crystals is proposed. The key features of the conduction mechanism are the following; (1) two kinds of defect states, H 2 XO 4 (+e) and XO 4 (-e) , are formed thermally by breaking of a hydrogen-bond, (2) the H 2 XO 4 (+e) defect state and the XO 4 (-e) defect state move coherently from an XO 4 tetrahedron to a distant XO 4 as the result of successive proton tunneling among hydrogen-bonds. The density of states and the mobility are calculated for the coherent motions of these defect states by the recursion method and the Kubo formula, respectively. The density of states shows the characteristic feature of the Bethe lattice, i.e., the twin peak structure due to self-similarity, while the conductivity is obtained as an order of a magnitude of 10 -3 Ω -1 cm -1 at the ferroelastic transition temperature, consistent with experiments.